<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <form id="f1">
        <input type="text" id="username" />
        <input type="password" id="pwd"/>
        <input type="submit" value="注册" id="sub" />
    </form>
</body>
<script>
    var f1 = document.getElementById("f1");
    var username = document.getElementById("username");
    var pwd = document.getElementById("pwd");
    var sub = document.getElementById("sub");
    f1.onsubmit=function(){
        alert("表单提交");
    }
    username.onblur=function(){
        var val = username.value;
        if(val.length<3){
            alert("用户名过短");
        }
    }
    pwd.onfocus=function(){
        alert("输入密码吧！");
    }
    
</script>
</html>